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An Olympus 35mm film camera with telefoto zoom lens.

35mm Film cameras

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by Chris Woodford. Last updated: October 7, 2017.

Snap a photo with your camera, cellphone, or MP3 player and you have a piece of digital information you can use in all kinds of different ways: you can instantly email it to a friend, upload it to a website, or edit it on your computer. Only a few years ago, this sort of thing wasn't possible because cameras worked differently: they were entirely analog, capturing pictures as patterns of light and dark using chemically treated reels of plastic film, and a photograph took hours (or even days) to appear in your hand. Although some professional photographers still use film cameras, most of us have long since abandoned them to history, along with steam engines and the telegraph. That's a shame, because they can actually teach us quite a bit about chemistry, physics, and the science of light. Let's take a closer look at how they worked!

Photo: A typical 35mm film camera dating from the mid-1990s. This one's made by Olympus and includes a motorized zoom lens that extends up to 80mm (about 3in). The plastic window at the top houses the autofocus system and the viewfinder and the white rectangle on the right (as we look at the camera) is the flash.

What is photography?

Original design of the Kodak camera as sketched out by George Eastman in his US patent 388,850 in 1888.

Artwork: Before digital cameras came along, photography involved capturing light rays on silver-based film. Light entered the front of the camera (yellow) through the aperture and lens (red) and hit a piece of film (green) wound out from the spools at the back (brown). Originally, photography was a very specialized and skilful business— until this little invention came along and allowed anyone and everyone to take photos. It's George Eastman's original Kodak camera, as illustrated in his "US Patent #388,850: Camera" granted on September 4, 1888. Artwork courtesy of US Patent and Trademark Office with our coloring and labeling added for clarity.

Articles like this typically open by noting that the word photography comes from two Greek words, photos (light) and graphos (writing)—so photography effectively means "writing with light." But that's just a metaphor. We see objects because they either emit light (like the Sun) or reflect it off their surface (like the Moon) in rays that zoom into our eyes in perfectly straight lines. Much like the human eye, a camera captures rays of light charging in through a lens at the front. But the crucial difference between a camera and your eye is that a camera makes a permanent copy of what it sees.

"Photography" is a bit of a misnomer: it's not really true to say that light rays "write" or "draw" in any sense. They don't move back and forth, thoughtfully scribbling out a picture like an artist's hand. So what does happen? It always helps to think like a scientist! Remember that light is a form of energy; what a camera actually does is permanently capture the energy falling on a small, (two-dimensional) surface inside it. In a digital camera, that happens because there's an electronic light-detector chip (either a CCD (charge-coupled device) or an alternative technology called a CMOS chip) immediately behind the lens, which converts the light energy into electricity. In a traditional camera, there's no chip; instead, the incoming energy is captured by a piece of plastic that is sensitive to light, better known as the film. The light energy leaves a permanent trace by causing a chemical and physical transformation of the film. (Incidentally, light doesn't just mean visible light: you can, in theory, make a photograph from any kind of incoming light: infrared, ultraviolet X-rays, or whatever you wish.)

How does a film camera work?

Inside the open back of a film camera.

Photo: You can't easily open up a digital camera to see how it works, and even if you do, you don't learn that much. But film cameras reveal their secrets more freely. Open the back and you can see the spools (left and right) where the film goes. In the middle, you can see the back of the shutter mechanism and the lens in front of it. The viewfinder is the little clear plastic window at the top.

The key features of a film camera are:

How to take a photo with a film camera

A camera with a large 400mm telephoto lens screwed on the front.

Photo: Need to take a photo at long range? Try a telephoto lens like this one, which is 400mm (~15 inches) long! Photo by Angela M. Virnig courtesy of US Navy.

When you're ready to take a photo, you point the camera at your subject, click a button, and the shutter opens briefly, allowing light rays to pass through the aperture and strike the film, usually for a fraction of a second before it closes up again. The incoming light rays cross over as they enter, with rays from the top of the object ending up on the bottom of the film and vice-versa, thus producing an upside-down (inverted) image on the film.

Film is very sensitive to light: only a tiny amount of light energy is needed to make a photograph and too much light will destroy it. To produce a perfect photo, you have to let exactly the right amount of light hit the film, which is called the exposure. The exposure depends on two factors: how long the shutter is open (the shutter speed) and how widely it's open (the aperture). Shutter speed is measured in seconds (anything from about 1/10,000 second to 30 seconds); aperture is measured in units called f-stops (or just "stops" for short), such as f/4 and f/8, with higher f numbers meaning more light is let in.

Automatic, compact, "point-and-shoot" cameras produce a reasonable image with the click of a single button: they use photocells (electronic light sensors) to automatically adjust the shutter speed and aperture and fire out invisible infrared or ultrasound beams to set the focus automatically as well. Although sophisticated professional cameras often have automatic controls, they also allow completely manual operation: before you can take a photo, you have to adjust the focus, set the exposure time, and adjust the size of the aperture. With manual cameras, you have to adjust the exposure time and aperture setting to compensate for one another, because both of them affect the amount of light reaching the film.

Other features

Most film cameras also have a viewfinder (so you can see how your photograph will appear), a xenon flash lamp (which adds enough extra light energy to activate the film, even in dark conditions), and self-timer mechanism (so you can photograph yourself without anyone's help). Inexpensive cameras generally have a viewfinder mounted to one side and above the main lens, so the image you compose is only an approximation of what you'll see on the final photograph. Professional cameras use a system called SLR (single lens reflex), in which prisms and mirrors allow you to look through the actual lens of the camera and see an exact replica of the photo you'll take. Most digital cameras (even inexpensive ones) produce a faithful copy of the final image, like SLRs, because the image you see on the LCD screen at the back is produced by the CCD or CMOS chip that captures the final photograph.

Labeled artwork showing the component parts of a typical SLR camera.

Artwork: How an SLR camera works: 1) Light enters at the front and passes through the lenses (2) and iris diaphragm (3), which has metal blades that open and close to let in more or less light. Inside the camera, the light bounces off a hinged mirror (4) and shoots up into a penta-prism (5, five-sided prism), which bounces it into the viewfinder (6) and your eye. When you press the shutter (not shown), the mirror (4) flips down out of the way and the light from the lens (dotted line) passes straight through to the back of the camera, hitting the film instead (7). This type of design ensures that the image you see through the viewfinder is exactly like the image captured on the film.

While inexpensive point-and-shoot cameras generally have fixed lenses, professional SLRs are designed so you can unscrew one lens and screw in longer or shorter ones, as necessary, according to what you want to photograph. Lenses generally range from about 20mm (~0.8 inch) wide-angle lenses (for photographing something relatively wide and quite close) to 800mm (30 inches) or so ("elephant gun", telephoto lenses for taking very distant shots of quite narrowly defined areas). In between these two extremes, a typical everyday lens is about 50mm (2 inches) long. Many modern cameras have zoom lenses, usually powered by small electric motors, that can be moved back and forth between two fixed points to provide a whole range of different magnifications.

How does photographic film work?

Many materials are sensitive to light. Leave a piece of white office paper in your window for a few weeks and you might well find it turns yellow; plastics that start off white or clear also have a habit of turning yellow or going foggy ("photodegrading") when they've been exposed to light for a while. The dyed colors in cotton clothes and fabrics will also fade in sunlight. And if you're Caucasian, even your skin may change color after a few hours or days on the beach. But you can't really use paper, plastic, cotton, or skin to capture a picture!

Photographic film is plastic (or sometimes paper) that's coated with an emulsion made from microscopically tiny crystals of silver salts suspended in gelatin (a jelly-like substance found in sweets such as wine gums). The silver salts are compounds of silver and halogens such as chlorine, iodine, and bromine, also called silver halides—and their useful feature is the way they begin to change into pure, metallic silver when light falls onto them. If lots of light hits them, they change much more dramatically than if less light hits. This is how the two-dimensional pattern of light rays entering through the lens of a camera from the world outside forms a kind of invisible, chemical trace (called a "latent" image) on the surface of photographic film.

Developing and printing photographic film

A photographic darkroom lit with green light.

Photo: A photographic darkroom is usually lit with dim green or red light to prevent damage to undeveloped film. Photo by Leah Stiles courtesy of US Navy.

Artwork showing the three steps of the photographic process: 1. exposure of the film; 2. developing the film with chemicals to make a negative; 3. shining light through the negative to make a positive print.

Artwork: The photographic process captures an image as a photograph in three main steps. 1: Exposure captures an inverted (upside down) latent (invisible) image on the film inside your camera. 2: Developing uses a series of chemicals to make the latent image visible and fix it permanently on the film in the form of a negative. 3: Printing produces a final photograph (a positive print) from the negative. You can make any number of prints from one negative.

A light-sensitive slice of plastic film with an image invisibly imprinted on it isn't much use to anyone. To turn it into a recognizable photo, you have to develop the film in a darkroom (usually lit with red or green light that doesn't affect the film). This involves dipping the film in a series of chemicals, which convert the latent image captured by the tiny silver halide crystals into a visible image formed of larger silver particles, and also makes that image permanent.

First, the film is dipped in an acidic solution called developer, which encourages more of the silver halide to convert to metallic silver and renders the latent image visible. To stop this process continuing indefinitely, and ruining the photo, the film then has to be dipped in an acidic solution called a stop bath to neutralize the developer. Once that's done, the image is made permanent by dissolving any remaining silver halide using a chemical solution known as hypo (or fixer), before being rinsed clean in water and hung up to dry.

At this stage, the image, though visible, is still in a negative pattern, with light areas looking dark and vice versa. That's why developed pieces of film are called negatives. Once the film is developed, it's printed: broadly speaking, you shine a light through the negative so it casts a shadow onto photo-sensitive paper and turns the negative film into a recognizable photograph called a (positive) print. You can make any number of prints from a single negative, which is one of the great advantages of this slightly laborious, "positive-negative" photographic process. By adjusting the distance between the negative and the paper you're printing on, and using lenses, you can also enlarge or reduce the size of a an image. The piece of equipment you use to do this is called an enlarger.

It's possible to develop and print films yourself, but most photographic laboratories have large electronic machines that automate the process completely, threading the film through a series of tanks filled with chemicals in the correct sequence, at just the right speed. Those big photo-printing machines you still sometimes see in the back of drug stores typically use a method of developing color film called the C-41 process.

A 35mm film negative of recycling dumpsters. A 35mm film positive of recycling dumpsters.
Photo: Left: A photographic color negative of some recycling dumpsters looks like this. Right: When it's printed, the colors are reversed and come out looking as you'd expect. I've simulated the effect of printing by reversing the colors digitally with a computer graphics package. Notice how the real-life red dumpster (in the center in the positive image on the right) turns green in the negative, while the yellow one (on the right in the positive image) turns blue in the negative.

Types of film

Most film is sold in light-tight cartridges that you snap into your camera. Inside, the cartridges contain a long reel of plastic film separated into one, two, or three dozen rectangular frames that measure 24mm x 36mm (this standard size is called 35mm film). The top and bottom of the reel is punched with little holes so each section of the film can be wound out of the way after a photo is taken, releasing an unexposed frame ready for the next photo. (Cameras generally have either a spool mechanism slowly wound by hand or automatically and very quickly wound by an electric motor.)

A piece of 35mm film showing four frames.

Photo: A typical piece of 35mm color photographic film. You can just about make out that there are four frames here, side by side. So a roll of film with 36 frames on it would be about nine times this length. Note the sprocket holes at the top and bottom for winding the film accurately through the camera.

There are numerous different kinds of film designed for taking different kinds of photo. Black and white film is sensitive only to the presence or absence of light, so it shows images only as shades of gray. Color film effectively works the same way as black and white only with three separate layers, one sensitive to blue, one sensitive to green, and one sensitive to red light. Films are also designed to work in widely different light conditions: generally speaking, you need to use a fast film (one that forms an image with relatively little light) in dark, indoor conditions and a slow film (one that needs more light) in bright, outdoor conditions. The film speed is indicated by a number called the ISO rating: ISO numbers of 100 are slow, 400 or more are fast, and 200 are good for general-purpose photography. Fast films generally produce grainier, more blurry images than slow ones so, as a rule, photographers never use a faster film than is absolutely necessary. Modern cameras automatically detect film speed using a system called DX coding, which simply involves the camera reading a barcode printed on the film container. For no real reason other than convention, good digital cameras also tend to use ISO ratings to indicate how quickly photographs are taken in different light conditions.

A brief history of photography

Louis Daguerre

Photo of Louis Daguerre courtesy of US Library of Congress.

Who invented cameras? Here's a brief history of the key moments in the development of film-based photography:

A Soho Cadet portable camera from about 1930.

Photo: Right: If you'd been a keen amateur photographer in the 1930s, you might have used a compact camera like this Soho Cadet (made in London, England), which packs into a sturdy outer case made from tough Bakelite plastic. The front of the case hinges down to reveal the camera mechanism inside. You pull on the lens and it extends out from the case on a leather bellows. You can focus the camera very crudely by pulling the bellows out from the case slightly more or slightly less, so adjusting the distance between the lens and the film. You can also crudely adjust the exposure with a slider control just above the lens. You can just make out the little viewfinder on the top right of the lens (as we look at it).

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Text copyright © Chris Woodford 2010, 2015. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2010/2015) 35mm Film cameras. Retrieved from http://www.explainthatstuff.com/how-film-cameras-work.html. [Accessed (Insert date here)]

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